All good advice so far. But I'll add.....
Show Process!!!
I don't think enough people do this. Employers will hire you for your mind and how it works. I layed out a book filled with scans of thumbnails, messy scribbled on paper, napkins, whatever. It usually ends up being a great conversation piece and allows me a chance to better explain my ideas and thinking. Show them all the critical creative steps in the beginning of the process. Conceptual ability is what can set you apart from the other 300 people applying for the job.
Also I'd say show your work on boards. I know that a lot of people like to show their portfolio in book form these days, but this isn't practical when you're interviewing with 5 people at once. They need something to pass around. Books are great for freelance though!